Neutral Hydrogen Optical Depth near Star-forming Galaxies at z ≈ 2.4 in the Keck Baryonic Structure Survey

Creators: Rakic, Olivera; Schaye, Joop; Steidel, Charles C.; Rudie, Gwen C.

Abstract

We study the interface between galaxies and the intergalactic medium by measuring the absorption by neutral hydrogen in the vicinity of star-forming galaxies at z ≈ 2.4. Our sample consists of 679 rest-frame UV-selected galaxies with spectroscopic redshifts that have impact parameters <2 (proper) Mpc to the line of sight of one of the 15 bright, background QSOs and that fall within the redshift range of its Lyα forest. We present the first two-dimensional maps of the absorption around galaxies, plotting the median Lyα pixel optical depth as a function of transverse and line-of-sight separation from galaxies. The Lyα optical depths are measured using an automatic algorithm that takes advantage of all available Lyman series lines. The median optical depth, and hence the median density of atomic hydrogen, drops by more than an order of magnitude around 100 kpc, which is similar to the virial radius of the halos thought to host the galaxies. The median remains enhanced, at the >3σ level, out to at least 2.8 Mpc (i.e., >9 comoving Mpc), but the scatter at a given distance is large compared with the median excess optical depth, suggesting that the gas is clumpy. Within 100 (200) kpc, and over ±165 km s^(–1), the covering fraction of gas with Lyα optical depth greater than unity is 100^(+0)_(–32%) (66% ± 16%). Absorbers with τ_(Lyα) > 0.1 are typically closer to galaxies than random. The mean galaxy overdensity around absorbers increases with the optical depth and also as the length scale over which the galaxy overdensity is evaluated is decreased. Absorbers with τ_(Lyα) ~ 1 reside in regions where the galaxy number density is close to the cosmic mean on scales ≥0.25 Mpc. We clearly detect two types of redshift space anisotropies. On scales <200 km s^(–1), or <1 Mpc, the absorption is stronger along the line of sight than in the transverse direction. This "finger of God" effect may be due to redshift errors, but is probably dominated by gas motions within or very close to the halos. On the other hand, on scales of 1.4-2.0 Mpc the absorption is compressed along the line of sight (with >3σ significance), an effect that we attribute to large-scale infall (i.e., the Kaiser effect).

Additional Information

© 2012 American Astronomical Society. Received 2011 September 20; accepted 2012 March 22; published 2012 May 9. Based on data obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and NASA, and was made possible by the generous financial support of the W. M. Keck Foundation. We are very grateful to Milan Bogosavljević , Alice Shapley, Dawn Erb, Naveen Reddy,Max Pettini, Ryan Trainor, and David Law for their invaluable contributions to the Keck Baryonic Structure Survey, without which the results presented here would not have been possible. We also thank Ryan Cooke for his help with the continuum fitting of QSO spectra, and we thank the anonymous referee for a careful reading of the manuscript and for valuable suggestions. This work was supported by an NWO VIDI grant (O.R., J.S.), by the U.S. National Science Foundation through grants AST-0606912 and AST-0908805, and by the David and Lucile Packard Foundation (C.C.S.). C.C.S. acknowledges additional support from the John D. and Catherine T. MacArthur Foundation and the Peter and Patricia Gruber Foundation. We thank the W.M. Keck Observatory staff for their assistance with the observations. We also thank the Hawaiian people, as without their hospitality the observations presented here would not have been possible.

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